30,807 research outputs found
A numerical study of detonation diffraction
An investigation of detonation diffraction through an abrupt area change has been carried out via a set of two-dimensional numerical simulations parameterized by the activation energy of the reactant. Our analysis is specialized to a reactive mixture with a perfect gas equation of state and a single-step reaction in the Arrhenius form. Lagrangian particles are injected into the flow as a diagnostic tool for identifying the dominant terms in the equation that describes the temperature rate of change of a fluid element, expressed in the shock-based reference system. When simplified, this equation provides insight into the competition between the energy release rate and the expansion rate behind the diffracting front. The mechanism of spontaneous generation of transverse waves along the diffracting front is carefully analysed and related to the sensitivity of the reaction rate to temperature. We study in detail three highly resolved cases of detonation diffraction that illustrate different types of behaviour, super-, sub- and near-critical diffraction
On the Cut-off Estimate in Lifshitz Five Dimensional Field Theories
We analyze if and to what extent the high energy behaviour of
five-dimensional (5D) gauge theories can be improved by adding certain higher
dimensional operators of "Lifshitz" type, without breaking the ordinary
four-dimensional Lorentz symmetries. We show that the UV behaviour of the
transverse gauge field polarizations can be improved by the Lifshitz operators,
while the longitudinal polarizations get strongly coupled at energies lower
than the ones in ordinary 5D theories, spoiling the usefulness of the
construction in non-abelian gauge theories. We conclude that the improved
behaviour as effective theories of the ordinary 5D models is not only related
to locality and 5D gauge symmetries, but is a special property of the standard
theories defined by the lowest dimensional operators.Comment: 18 pages, one appendix; v2: minor improvements, to appear in Phys.
Rev. D; v3: one typo fixed, incorrect argument at the end of section 5
removed, acknowledgments added, conclusions unchanged, supersedes published
version v
Invariants of Combinatorial Line Arrangements and Rybnikov's Example
Following the general strategy proposed by G.Rybnikov, we present a proof of
his well-known result, that is, the existence of two arrangements of lines
having the same combinatorial type, but non-isomorphic fundamental groups. To
do so, the Alexander Invariant and certain invariants of combinatorial line
arrangements are presented and developed for combinatorics with only double and
triple points. This is part of a more general project to better understand the
relationship between topology and combinatorics of line arrangements.Comment: 27 pages, 2 eps figure
Concatenating Variational Principles and the Kinetic Stress-Energy-Momentum Tensor
We show how to "concatenate" variational principles over dierent
bases into one over a single base, thereby providing a unied Lagrangian
treatment of interacting systems. As an example we study a Klein{
Gordon eld interacting with a mesically charged particle. We employ
our method to give a novel group-theoretic derivation of the kinetic
stress-energy-momentum tensor density corresponding to the particle
Coherence in parametric fluorescence
We investigate spontaneous four wave mixing (SFWM) in a single-channel
side-coupled integrated spaced sequence of resonators (SCISSOR). Analytic
expressions for the number of photon pairs generated, as well as the biphoton
wave function (joint spectral amplitude) describing the pairs, are derived and
numerically computed for different pump pulse durations and numbers of ring
resonators. In the limit of a long input pump pulse, we show a strong analogy
between super-linear scaling of generation efficiency with respect to the
number of rings in the structure and Dicke superradiance. More generally, we
discuss in detail the factors that influence the shape of the biphoton wave
function, as well as the conditions for observing super-SFWM
Affine parameterization of the dark sector: costraints from WMAP5 and SDSS
We study a set of universe models where the dark sector is described by a
perfect fluid with an affine equation of state , focusing
specifically on cosmological perturbations in a flat universe. We perform a
Monte Carlo Markov Chain analysis spanning the full parameter space of the
model using the WMAP 5 years data and the SDSS LRG4 survey. The affine fluid
can either play the role of a unified dark matter (UDM), accounting for both
dark matter and a cosmological constant, or work alongside cold dark matter
(CDM), as a form of dark energy. A key ingredient is the sound speed, that
depends on the nature of the fluid and that, for any given background model,
adds a degree of freedom to the perturbations: in the barotropic case the
square of the sound speed is simply equal to the affine parameter ; if
entropic perturbations are present the effective sound speed has to be
specified as an additional parameter. In addition to the barotropic case, we
consider the two limiting cases of effective sound speed equal to 0 or 1. For
our UDM model is equivalent to the standard CDM with
adiabatic perturbations. Apart of a trivial subcase, all models considered
satisfy the data constraints, with quite standard values for the usual
cosmological parameters. In general our analysis confirms that cosmological
datasets require both a collisionless massive and cold component to form the
potential wells that lead to structure formation, and an effective cosmological
constant that drives the late accelerated expansion.Comment: 10 pages, 9 figure
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